This invention relates to an air/fuel ratio control method for feedback control of the air/fuel ratio of an air/fuel mixture being supplied to an internal combustion engine in response to concentration of an ingredient in the exhaust gases emitted from the engine, and more particularly to a method of this kind which enables positive control of the air/fuel ratio of the air/fuel mixture to values best suited for actual operating conditions of the engine or values close thereto, when the engine is operating in particular operating regions, to thereby improve the operational stability of the engine, as well as to eliminate a lag in the feedback control of the air/fuel ratio of the air/fuel mixture to a required value, which occurs when the operating condition of the engine is temporarily changed to a particular operating region from the feedback control region and returned to the latter, caused by the speed changing operation of the transmission gear.
A fuel supply control system adapted for use with an internal combustion engine, particularly a gasoline engine has been proposed e.g. by U.S. Pat. No. 3,483,851, which is adapted to determine the valve opening period of a fuel injection device for control of the fuel injection quantity, i.e. the air/fuel ratio of an air/fuel mixture being supplied to the engine, by first determining a basic value of the valve opening period as a function of engine rpm and intake pipe absolute pressure and then adding to and/or multiplying same by constants and/or coefficients being functions of engine rpm, intake pipe absolute pressure, engine cooling water temperature, throttle valve opening, exhaust gas ingredient concentration (oxygen concentration), etc., by electronic computing means.
Also, in an engine having a three-way catalyst arranged in its exhaust system, it is generally employed to control the air/fuel ratio of the mixture to a theoretical mixture ratio in a feedback manner responsive to the output of an exhaust gas concentration sensor which may be represented by an O.sub.2 sensor, arranged in the exhaust system of the engine, to obtain the best conversion efficiency of unburned hydrocarbons, carbon monoxide and nitrous oxides in the exhaust gases emitted from the engine. However, this feedback control based upon the output of the exhaust gas sensor cannot be applied when the engine is operating in a particular operating condition such as engine idle, wide-open-throttle, mixture-leaning, and deceleration where the air/fuel ratio of the mixture needs to be controlled to a value different from the theoretical mixture ratio.
Therefore, in the case of applying the above exhaust gas concentration-based feedback to the aforementioned fuel supply control system using coefficients, etc., it is necessary to carry out open-loop control when the engine is operating in a plurality of particular operating conditions, by using coefficients having predetermined values corresponding to the respective particular operating conditions, so as to achieve desired predetermined air/fuel ratios best suited for engine operation under the above respective particular operating conditions.
It is thus desirable that the predetermined air/fuel ratio corresponding to the particular operating condition can be achieved with certainty by means of open-loop control. However, as a matter of fact, the actual air/fuel ratio can sometimes have a value different from the desired predetermined value due to variations in the performance of various sensors for detecting the operating condition of the engine and a system for controlling or driving the fuel quantity metering or adjusting means. In such event, it is impossible to obtain required operational stability and driveability of the engine.
To overcome such disadvantage, an air/fuel ratio feedback control system has previously been proposed by the applicants of the present application in Japanese Patent Provisional Publication (Kokai) No. 57-210137, in which the air/fuel ratio of an air/fuel mixture being supplied to the engine is controlled to required values or values close thereto by the use of a first coefficient which has a value variable in response to the output of an ingredient concentration detecting means that detects the concentration of an ingredient in the exhaust gases emitted from the engine, while the engine is operating in a feedback control region, and by the use of a second coefficient which is a mean value of values of the first coefficient applied during operation of the engine in the feedback control region, while the engine is operating in a particular operating region other than the feedback control region, to thereby improve the operational stability, driveability, emission characteristics, etc. of the engine.
However, according to this proposed system, when the engine is operating in the feedback control region, there can occur a temporary transition of the operation of the engine to a particular operating region upon operating the transmission gear and then returned to the feedback control region upon completion of the operation of the transmission gear. On such occasion, if the aforementioned second coefficient is used for controlling the air/fuel ratio of the air/fuel mixture simultaneously upon the above transition of the operation of the engine to the particular operating condition, the air/fuel ratio feedback control is resumed with the value of the same second coefficient applied as an initial coefficient value immediately when the operation of the engine is returned to the feedback control region. Consequently, there occurs a lag between the resumption of the feedback control and the time a required air/fuel ratio is actually obtained by the same feedback control, which is appropriate for the operating condition of the engine in the feedback control region, resulting in deterioration of the emission characteristics and wasteful fuel consumption of the engine.